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stacy said:
Porosity
In a normal, undamaged hair shaft, very little water can get either into or out of the cortex. This is because the cuticle covering the cortex is intact, and is then almost (but not quite) waterproof. Shampoos do not damage the
cuticle. When hair is permed or tinted, however, the chemicals have to penetrate the cortex in order to react with the keratin inside it. Increasing the temperature, or applying an alkaline lotion, separates the scales of the cuticle enough to allow the chemicals to pass through. After the processing is finished the scales gradually close up again.
But if hair is processed too many times the cuticle scales may never return to their original tightness and the protection they once offered is lost. The cuticle can also be damaged in the same way by too much blow drying, curling irons that are too hot, and the effects of wind and sun. The hair becomes increasingly porous, and water can then pass in and out of the cortex.
Over-porous hair is dry, and tends to develop split ends. The damaged cuticle is fragile, and the damage worsens as time goes by. The greater the damage, the more the cortex swells with water whenever the hair is washed, but the more water it loses when it dries. The repeated wetting and drying of the cortex gradually weakens the hair.
Sun, previous chemicals, blow drying, alkaline shampoos, inactive or low active thyroid gland and diet low in protein all contribute to porous hair.
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Thanks Stacy, very informative!! /images/graemlins/smile.gif